Scientists in the Mucosal Immunity Section have long been interested in the regulation of Th1/Th2 differentiation, particularly as the latter relates to the pathogenesis of autoimmunity and the response to infectious agents. In work previously accomplished in this area we developed transgenic mice whose T cells constitutively express the IL-12R beta2 chain under a CD2 promotor/enhancer. Subsequently, we interbred these transgenic mice with mice expressing a transgene for the TCR recognizing ovalbumin and performed in vitro studies of the capacity of T cells from the double transgenic mice so obtained to undergo T cell differentiation into Th1 and Th2 T cells. We found that contrary to accepted ideas, when T cells from the double transgenic mice are stimulated in the presence of IL-4 and IL-12, they undergo Th2 differentiation in spite of persistent expression of the IL-12R beta2 chain and competent IL-12 signaling (STAT4 phosphorylation). We thus established that down-regulation of the beta2 chain by IL-4 is not the determinative factor allowing IL-4 to direct Th2 T cell differentiation.In the current period we tested this conclusion by performing in vivo studies of the capacity of Balb/c mice bearing the transgenic IL-12R beta2 chain to develop a healing phenotype when subjected to Leishmania major (L. major) infection. This approach was based on the now well-known observation that Balb/c mice develop a non-healing infection because they mount an early IL-4 response to L. major antigen which then directs T cell differentiation along a Th2 pathway. This has been assumed to be due to IL-4-induced down-regulation of the IL-12R beta2 chain and thus the extinction of IL-12 signaling. According to this assumption, therefore, if the IL-12R beta2 chain cannot be down-regulated the Balb/c mouse should develop a Th1 response and a healing phenotype. In initial studies we showed that CD4+/IL-12R beta2 chain+ expressing cells producing IFN-gamma; thus cells having the beta2 chain are in fact the Th1 T cells responsible for the healing phenotype. Next, we demonstrate that Balb/c background mice bearing an IL-12R beta2 chain transgene and thus incapable of down-regulating the IL-12 receptor in the presence of IL-4, nevertheless manifested a non-healing phenotype similar to littermate non-transgenic controls. Furthermore, we found that such transgene mice still express a non-healing phenotype when treated with IL-12 seven days after initiation of infection. These studies thus show that while CD4+/IL-12R beta2 chain+ cells are important components of the Th1 responses, maintenance of beta2 chain expression in the face of IL-4 secretion is not sufficient to change a Balb/c/Th1 response to a Th2 response in vivo which would thus allow Balb/c mice to manifest a healing phenotype to L. major infection. The IL-4-induced commitment to Th2 differentiation in vivo does not depend on down-regulation of the IL-12R beta chain.